1. Field of the Invention
The present invention generally relates to a spindle motor suitable for a disk drive that can be utilized as an external memory of a computer system, such as a magnetic disk or magneto-optical disk. More specifically, the present invention is concerned with a whole construction of a disk drive having a housing of a credit card type, a circuit assembly thereof and the spindle motor.
2. Description of the Related Art
Generally, disk drives, e.g., magnetic disk drives, having at least one magnetic disk utilized as a recording medium, have been in practical use in various areas including computer systems as non-volatile memory devices. Further, in recent years, improvements in the technology of the magnetic disk drives, such as increasing the magnetic recording density of the magnetic disk, have been realized, leading to down-sizing of magnetic disk drives per se. On the other hand, computer systems, etc., have been becoming more compact, lighter in weight and lower in power consumption, as represented by a portable personal computer, owing to recent rapid development of microelectronics.
Though down-sizing of the technology of the magnetic disk drive has progressed recently as described above, the dimensions of related apparatuses are still too large, the apparatuses are too heavy and the power consumption thereof is too high if a magnetic disk having a diameter of 2.5 inches is utilized. Therefore, it may be difficult for the current magnetic disk drives to be applied to the above portable personal computers for which compactness, lighter weight and lower power consumption are required. To meet this requirement, a magnetic disk drive utilizing a magnetic disk with a diameter of 1.89 inches has been recently announced in public. The magnetic disk drive surely has smaller dimensions than the magnetic disk drive with a diameter of 2.5 inches. However, in such a magnetic disk drive comprising a magnetic disk with a diameter of 1.89 inches, down-sizing of the magnetic disk drive has been attempted by using the prior art without making any improvements therein. Therefore, problems exist in that the dimensions of the above magnetic disk drive, especially the thickness or height thereof, are still too large for the disk drive to be practically used as a portable device (nowadays, it is generally well known that the lower limit of the thickness thereof is as large as 10 mm).
Further, a modular unitary disk file subsystem has been disclosed in U.S. Pat. Nos. 4,639,863 and 4,860,194, wherein an elongated printed circuit board is directly attached to the side of a housing including a head and disk assembly so as to attain thinner dimensions. However, a concrete thickness value in such a construction is not disclosed in these prior art disclosures. Moreover, even though the thickness of the disk drive can be reduced successively, a new problem would occur in that the area of the disk drive including the printed circuit board and the housing is larger than usual.
Taking these conditions into account, in a known portable personal computer or the like currently in use, an integrated circuit (IC) memory card is provisionally utilized, rather than a magnetic disk, so that the required dimensions and weight can be attained. The specifications of this IC memory card have been standardized recently {the standard specification of JEIDA (Japan Electronic Industry Development Association) and PCMCIA (Personal Computer Memory Card International Association)}, where the thickness or height of the card is defined as 5 mm or 3.3 mm. A card satisfying these standard specifications is sufficiently thin and sufficiently light, and therefore the above card can be suitable for application to a portable personal computer, etc., in view of dimensions and weight.
However, presently, there are two significant disadvantages in the above IC memory card as follows.
First, computer systems utilizing the IC memory card are extremely expensive. More concretely, the cost per megabyte thereof is several tens of thousands of yen/MByte, which is several hundred times higher than a computer system utilizing the flexible disk drive and is several tens of times higher than that utilizing the hard disk drive (i.e., magnetic disk drive).
Second, the whole storage capacity of a computer system utilizing the above IC memory card is not always sufficient to comply with current user requirements. Currently, an IC memory card having a storage capacity of approximately 1 MByte is widely used. In the future, the storage capacity of the IC memory card will be increased up to the order of several MBytes to 10 MBytes. On the other hand, presently, in the ideal portable personal computer, a memory system having more than 40 MBytes is actually required. Accordingly, the computer system utilizing the above IC memory card cannot substantially satisfy the current requirements for storage capacity. Moreover, in the near future, the above storage capacity required by users will be expected to increase more and more. Therefore, it will be difficult for the storage capacity of the IC memory card to catch up with the required storage capacity, even taking into account the progress of IC memory technology.
As described above, if a magnetic disk drive according to the prior art is to be utilized for a portable personal computer, it will be sufficient in respect to cost and storage capacity thereof, but it is not sufficient in respect to dimensions, weight, power consumption and durability against mechanical shock. On the contrary, the IC memory card currently utilized for portable personal computers is sufficient in respect to dimensions, weight, power consumption and durability against mechanical shock, however, the cost of the IC memory card is too expensive and the storage capacity thereof is not always satisfactory for the users. Therefore, in order to realize a suitable portable personal computer, a memory device that has the advantages of both the magnetic disk drive and the IC memory card is strongly desired.
As a strategy for overcoming the abovementioned difficulties, it is considered effective to utilize the specifications of a type III PCMCIA. In this type III PCMCIA, the same dimensions as type I and type II are defined in respect to plane directions, while the thickness dimensions are allowed to be a maximum value of 10.5 mm. If one connector conforming to the type III of PCMCIA is provided, a card having thickness of 10.5 mm can be inserted into two different kinds of slots of the type I and type II arranged in a vertical direction.
As described above, if the thickness dimension is specified to be 10.5 mm, a disk drive of a card type can be realized using the prior art without any improvements. Actually, a device having a thickness of 10.5 mm has already been announced. However, down-sizing of the device is required for personal computers, particularly notebook type personal computers, and therefore, a structure such that the two slots are arranged along a vertical dimension may be disadvantageous in terms of down-sizing. On the other hand, in palm top type personal computers, only one slot can be provided in each personal computer. In other words, it is now difficult for memory devices in all areas that IC memory cards are utilized to be replaced with magnetic disk drives. Therefore, it is strongly desired that magnetic disk drives having outer dimensions conforming to type I or type II (i.e., disk drives with a thickness equal to or less than 5 mm) be realized.
In order to produce the above magnetic disk drives, it is required to improve a spindle motor for driving a disk. As the size of disks is reduced, the diameter of the center hole of the disks is reduced (the diameter of the center hole of a disk with a diameter of 1.89 inches is 12 mm). Hence, it is strongly desired to diminish the size of spindle motors for use in disk drives.
The inventors note that various disk drives and spindle motors have been proposed in, for example, U.S. Pat. No. 5,025,355, U.S. Pat. No. 5,025,336, Japanese Laid-Open Patent Publication No. 3-212148 (corresponding to U.S. patent application Ser. No. 424,673), U.S. Pat. No. 4,980,587, U.S. Pat No. 4,996,613, U.S. Pat. No. 5,045,738, U.S. Pat. No. 5,138,209, Japanese Laid-Open Patent Publication Nos. 2-62425, 3-253240, 3-277157, or Japanese Laid-Open Utility Model Publication Nos. 1-86454 and 3-77269, or M. Iwana, "Technical Trend in spindle motors for HDD", Denpa Sinbunsha, Sep. 3, 1992. The inventors also note the following documents: Japanese Laid-Open Patent Publication Nos. 57-109391, 58-157360, 59-76146, 59-123450, 61-192068, 61-236351, 63-18951, 1-315244, 2-193554, 3-15255, 3-108178 and 4-58753, and Japanese Laid-Open Utility Model Publication Nos. 54-64212, 60-25381, 60-42082, 60-66282, 60 -119886, 62-61184 and 62-70668.